Conventionally, four-wheel vehicles are equipped with directional wheels enabling them to change direction. The wheels are linked to the vehicle by means of pivot links and, in the case of directional wheels, an additional degree of freedom in rotation is added. When the directional wheels are drive wheels, universal joints make it possible to drive the wheels while making it possible to modify the orientation of the axis of the pivot link. This type of configuration does not make it possible to produce small turn radii. In other words, it is impossible to pivot the vehicle in place without speed.
Moreover, attempts have been made to produce a vehicle that has spherical wheels that can each pivot on itself. An example of such production is described in the patent application published under the number JP 2007-210576. This document describes a wheel comprising two hemispheres. The wheel is actuated by a horizontal motor shaft which rotationally drives a support bearing the two hemispheres. The latter are mounted on the support, each via a pivot link. The two pivot links are aligned. Their common axis is at right angles to the axis of rotation of the motor shaft. A vehicle, such as a robot, can be equipped with four wheels as described in the document JP 2007-210576. The axes of the motor shafts of four wheels are then arranged at right angles to one another. The wheels are then aligned in pairs. The driving of two wheels of a first pair makes it possible to move the vehicle in a direction at right angles to the common axis of that pair of wheels. For the second pair of wheels, the hemispheres revolve freely about their pivot link. A movement of the robot in a direction at right angles is obtained by driving the wheels of the second pair. Combined movements are of course possible. This makes it possible for the vehicle to be displaced in any direction and even to revolve on itself.
The drive shaft of a wheel penetrates into the wheel by an equatorial plane of the wheel, situated between the two hemispheres. The equatorial plane is defined by analogy to the terrestrial sphere. This plane separates the two hemispheres that can be likened to the north and south terrestrial hemispheres. The motor shaft must have a certain rigidity which imposes on it a minimum diameter. The two hemispheres are therefore separated by at least this diameter. In practice, a functional play preventing the hemispheres from rubbing against the motor shaft must be added to the diameter of the shaft. Two hemispheres each delimited by a plane are therefore obtained. The planes of the two hemispheres are parallel and arranged at a distance that cannot be reduced without the risk of impairing the rigidity of the motor shaft.
When the equatorial plane of the wheel is in vertical position relative to the ground, that is assumed horizontal, a discontinuity of the bearing of the wheel on the ground appears. More specifically, when the wheel is a drive wheel, the equatorial plane of the wheel comes, on each wheel revolution, into contact with the ground and leads to a ground bearing transition from one hemisphere to the other and therefore from a plane of one hemisphere to the other. In the transition through this discontinuity, a loss of grip may occur, the sphericity of the wheel is momentarily lost and, at high speed, a noise occurs on each transition.
In the French patent application FR 12-53981 filed on Apr. 30, 2012 in the name of the applicant, another spherical wheel, closely related to that described in the document JP 2007-210576, was proposed. This other spherical wheel reduces the drawback of the discontinuity in the transition through the equatorial plane of the wheel by making the planes delimiting the two hemispheres secant. In practice, the term “hemisphere” can no longer be used and is replaced by the term “cap” to define the two wheel parts of which each of the surfaces follows the spherical surface of the wheel. The “caps” are each articulated by means of a pivot link relative to the motor shaft of the wheel. The two caps are each delimited by a plane and the two planes are secant. In other words, the axes of the two pivot links are no longer aligned.
These two implementations exhibit a singularity when the axis of one of the pivot links is at right angles to the ground. In this configuration, if the vehicle using the wheel has a speed vector that is not at right angles to the driving axis of the wheel concerned, the latter slips at its point of contact with the ground. To avoid this slip, each cap (or hemisphere) comprises a caster arranged in the extension of the pivot link of the cap concerned and that ensures a rolling on the spherical surface. The casters have a degree of freedom in rotation about an axis at right angles to the driving axis of the wheel. This rotational movement is sufficient to avoid the slipping of the wheel in a singularity configuration. The casters each have a rolling line which follows the spherical surface of the wheel.
Both in the document JP 2007-210576 and in the French patent application FR 12-53981, the casters are as small as possible in order to keep the cap surfaces as large as possible for the wheel. In effect, the wheel can be a drive wheel only when it is in contact on the ground on the surface of its caps. There is therefore an incitement to reduce to the maximum the dimensions of the casters, notably to reduce as far as possible the circular opening produced in each of the caps to allow the respective casters to pass.
Tests performed in-house by the applicant have demonstrated that, on the transition through the singularity, the speed of rotation of the cap concerned changed very significantly, which presents a drawback because of the inertia of the cap. More specifically, for a constant linear speed of the vehicle, the speed of rotation of a cap in contact with the ground can change from a minimum value when the rotation of the cap takes place at the level of the plane which limits it to a maximum value when the rotation of the cap takes place at the level of the edge of the opening formed for the passage of the caster. Still for a constant linear speed of the vehicle, it is possible to have a sequence stringing together different points of contact with the ground:                contact with the ground on the plane delimiting the cap,        transition through the edge of the opening,        rolling on the caster,        transition again through the edge of the opening,        and finally return to the plane delimiting the cap.        
Upon the first transition through the edge of the opening, the speed of rotation of the cap about its pivot link takes place in a direction which has to be reversed at the moment of the second transition through the edge of the opening to avoid any friction of the cap on the ground. The inertia of the cap can interfere with this reversal of direction of rotation. This interference is all the greater when the dimensions of the opening are small because of the high speeds of rotation achieved by the cap. This interference is further amplified with an increase of the linear speed of the vehicle. In effect, this increase of linear speed tends to increase the speed of rotation of the cap and reduce the time available for the reversal of speed of rotation in the vicinity of the singularity. These abrupt changes of speed for the cap necessitate, as for the caster, a significant input of kinetic energy and can generate risks of friction between the wheel and the ground.